(define (lookup given-key set-of-records) 
  (cond ((null? set-of-records) #f) 
        ((= given-key (key (car set-of-records))) 
         (car set-of-records)) 
        ((< given-key (key (car set-of-records))) 
         (lookup given-key (left-branch set-of-records))) 
        (else (lookup given-key (right-branch set-of-records))))) 
